I noticed today that when I turn my lights on,my fuel gauge decides to read full.Doesnt matter if its side lights or headlights.
Cant for the life of me think what could be causing this,sounds like a short,and for the gauge to read full,it must be a negative short to the terminal coming from the sender,but when the lights come on,the light switch will be sending a positive feed,not a negative.
I havent removed the dash yet,will do tommorow.
Any ideas?

Must be playing silly buggers with the resister in the fuel sender

If you un plug the fuel gauge the sender should read full?

I cant for the life of me see it being anything to do with the sender in the tank,as that only works on the negative feed to the gauge via the resistor,and as the problem only occurs when the lights are on,that feed from the sender shouldnt change.Could be an internal gauge problem I suppose?

Unless the extra current being drawn is sending it haywire

poor earth somewhere

negative feed?
electricity comes from the battery through the gauge along the wire and earths via the variable resistor that is the tank sender.
if you pull the bullet next to the wheel arch and earth it you cut out the variable resistor and the guage reads full

Just do a continuity test between the light switch and the gauge see if there is anything there cause if theres a short that could cause it?

With the lights on the gauge is earthing through the lights, the lights have a lower resistance than the sender so the gauge reads full.

Somehow the wire from the gauge to the sender is connected to the feed to the light switch and the regular light feed is broken, couldn't tell you how though.